Drilling holes for gas, petroleum or geothermal exploration are typically 30 cm in diameter and approximately 2 km/1.5 miles long. These drillings are drilled with drilling strings of relatively light-weight drilling rods, screwed together, the joints being provided with hinges, of 9.14 m/30 ft or 13.72 m/45 ft length. As the drilling progresses, further drilling rods are added to the drilling string at the upper end of the drilling hole. At the lower drilling hole end of the drilling string there is usually a drill stem, the natural weight of which corresponds approximately to that of normal drilling rods placed end to end to a length of 300 m/1000 ft. The drill stem is equipped with a drill bit. Due to the weight of the arrangement and the rotating drive of the drilling string from the surface, the drill bit digs into the ground. Sometimes, drilling mud motors or drilling mud turbines are also used for the drill bit drive. Jetting mud or air is supplied to the drill bit from the surface through an axial bore in the drilling string. This fluid removes the depositions from the drilled hole via the hollow-cylindrical space between the outside wall of the rods and the wall of the drilled hole. Using a hydrostatic drill head, the gases of the ground formation are monitored or sometimes also cooling is applied to the drill bit.
Transmitting sensor data relating to parameters, such as pressure or temperature, which are detected by means of sensors at the drill string in the area of the bottom of the drilled hole, to the surface, has already been demanded for a long time. Various methods of this communication have been attempted, such as electromagnetic wave propagation via the ground formation, electrical transmission via an insulated conductor, pressure pulse propagation via the drilling mud and acoustic wave propagation via the metallic drilling string. Each of these methods has disadvantages such as, for example, relating to signal attenuation, environmental noises, high temperatures and the incompatibility with standard operating methods. The most widely used method commercially is that of information transmission by means of a pressure pulse via the drilling mud. However, damping mechanisms of the mud limit the transmission rate to approximately 2 bit/s to 4 bit/s.
A further reason for the transmission of the information via the drilling string is the result of the wish for automatic maintenance of a predetermined direction of advance. This is of importance mainly in the case of mining-related deep drilling with a drill pipe carrying the drilling bit and an outer pipe arranged rotatably on the drill pipe and provided with guide strips and press parts. Drilling rods for full and core drillings are also affected. In the case of deep drillings in above-ground and underground mining, the earth's attraction, the layering of earth formations, especially at the transition from hard to soft layers and vice versa, and also the external friction of the drilling tool and of the rods cause deviations from the predetermined direction of drilling. This applies both to core and full drillings. Especially in underground mining, where the starting and exit point of a drilling are often specified precisely, the drilling must maintain the desired direction. Such drillings are also called target drill holes. In the case of exploratory drillings for searching for unknown deposits, too, a straight-line course of the drilling is demanded. Exploratory drillings are carried out both in accordance with the core- and the full-drilling method. In the case of parallel drillings, too, such as, e.g., for dike reinforcement, the plumb line and parallelity are absolutely mandatory for reasons of later sealing of the dike installation.
For the straight-line course of the drilling, so-called target drilling rods, stabilizers or centering and guiding devices are installed. These are drilling rods with attached guide strips, the outer diameter of which corresponds to the drilling diameter and, following the advancing drilling tool, are intended to guide the latter concentrically. Target drilling rods have an inbuilt automatic vertical control which specifies and/or corrects the direction of drilling by utilizing the force of the earth's attraction and using the pressure of the jetting liquid. In the case of straight-line drillings both in accordance with the full- and the core-drilling method, intermediate measurements of direction are required even with lengths of up to 100 m, in order to align the drilling in accordance with the deviation found. This work is extremely time-consuming and expensive, especially in the case of very deep drillings or in the case of core- or full-drillings in accordance with the cable core-drilling method in which two different drilling rods and machine equipments are used. The straight-line drillings include horizontal, vertical and oblique drillings.
There is thus great interest in arranging corresponding measuring instruments as closely as possible to the drill head and/or at the drill bit of the drilling string in order to provide the correct measurement data online and in real time via the drilling string to processing facilities at the surface, in order to be able to react immediately, e.g. to deviations from the destination. Corresponding measuring instruments are, for example, inclination sensors such as inclinometers, deflectometers or pendulum plum-line meters. The transmission of measurement variables from sensors of other detection variables, i.e., other physical variables, is just as significant. In the text which follows, the prior art of facilities for transmitting information via drilling rods is acknowledged. According to the prior art, a multiplicity of types of facilities for transmitting information via drilling rods are used, depending on the field of application.
EP 1 225 301 A1 describes a “hollow drill pipe for transmitting information” consisting, on the one hand, of an electrically conductive hollow rod linkage which comprises on the inside a cylindrical layering of insulation, line and insulation, the line being exposed in each case at the ends of the rods to a length of L=0.8×D to 2.2×D for forming conductive rings and contacting the drilling mud and, on the other hand, of a drilling string of a number of rods and a drilling tool at the lower end, wherein, for the transmission of information, a first inner axial coil arrangement 1 at the lower end, which is suitable for receiving electrically alternating signals as information carriers, and a second inner axial coil arrangement 2 at the upper rod linkage for the signal reception, the signals being generated by the circulation of a current in a current loop which is formed by the conductive layer, the conductive rings, the internal mud, the wall of the rod linkage and the external mud, and wherein the current is generated by the signal which acts on the coil arrangement 1. Since the diameter D is between 2.5 cm and 11 cm, this is an extremely long and thin line both in the mechanical and in the electrical sense.
EP 1 213 440 A1 describes a “method for transmitting information along a drill string” and a device therefor, wherein each rod of the drilling rod linkage has an electrically conductive inner line for conveying the liquid which is under pressure, surrounded by an annular space filled with a fluid electrical insulator and an electorally conductive wall closed off to the outside, a first coil arrangement for inductively coupling-in being located in the vicinity of the closing lower end of the drilling rod linkage, arranged in the annular space, enclosing the inner line, and a second coil for inductive coupling-out being located in the vicinity of the upper end of the rod linkage in the same arrangement. A battery-fed measurement sensor is additionally located in the vicinity of the drill head, the signal of which sensor is supplied amplitude-processed and frequency-processed to the first coil, transmitted and received and processed by the second coil. The coils can also be switchably used as transmitting and receiving coils. A further transmitting coil is also provided in the vicinity of coil 2 and a further receiving coil in the neighborhood of coil 1. The arrangement represents electrically a coaxial line consisting of an inner conductor, cylindrical insulating layer and an outer conductor. The quality of the transmission is dependent on material, medium, rotational speed, length, amplitude and frequency.
EP 468 891 A1 describes a “dynamometric measuring assembly for drill pipe with radio transmission means” which has measurement sensors permanently connected to a rotatable shaft and a first electronic circuit for processing the signals provided by the measurement sensors, the signals being conducted to a stationary detection unit which is remote from a radio transmitter permanently mounted on the rotatable shaft and the device also having a radio receiver for receiving the signals transmitted by the detection unit, and the radio receiver has facilities for parameterizing or controlling the measuring device in response to the signals transmitted by the detection unit. The measurement sensors of the force measuring device are installed above ground as is the radio link so that the rotating radio source and the stationary sink remains as special technical feature.
WO 91 00 413 A1 describes a “device for force measurement for a drilling rod linkage” where the radio link described in EP 468 891 A1, having a similar structure of measurement sensors for the force measurement of the drill shaft, is replaced by a collector, corotating with the drilling rods, with fixed brush pick-off. Preceding and following electronic circuits are used for measurement value editing and processing. Since the force measurement does not take place at the location of the event, i.e., at the drilling head or in its immediate vicinity, the result of the measurement must be adapted to the actual conditions and post-processed, in any case.
EP 1 915 504 A1 describes a “Bidirectional drill string telemetry system for measurement and drilling control”, consisting of a drilling platform with drilling tower and a suspension with hook and rotary hinge for the drill string, a drilling table with follower rod, a pump for the drilling fluid which is conveyed from a pit via the rotary hinge into the interior of the hollow-cylindrical drilling rods and, emerging over the drill bit, is pressed upward as drilling mud between outer drilling string wall and drill hole, with electronics equipment below the rotary hinge which communicates wirelessly, on the one hand, with the building site control computer and, on the other hand, with the surface participant of the drilling string which forms an information-related termination of a network of drilling rods, the drilling rod linkages being wired over their length (wired drill pipe (WDP)) and having at their respective end in each case an inductive coupling to next drilling rod linkage and, dependent on the length of the drilling string, intermediate amplifiers, and an arrangement of control and measuring devices above the drill bit (bottom hole assembly (BHA)), and a motor controller, various logging (logging while drilling (LWD)) and measuring (measurement while drilling (MWD)) modules which terminate with an interface participant towards the wired drilling rods. The wired, inductively coupled rods which extend from the surface participant to the interface participant form the drilling string telemetry system.
U.S. Pat. No. 7,040,415 two further telemetry systems with their methods, wherein the drilling string data are picked up once via slip rings at the drilling platform and conveyed to the building site computer and, in another example, by means of a wireless transmission. The rod linkages are in each case equipped with two pairs of lines, the adapters connected between the rod linkages providing inductive coupling between the two pairs of lines.
EP 1 556 576 B1 describes a “Drill pipe having an internally coated electrical pathway” where hollow-cylindrical drilling rods connectable to one another by screwing are coated with insulation on the internal circumferential area and are then provided with electrically conductive coating on the cylindrical insulating area. The method is continued with a further insulating layer on the conductive layer and a further conductive layer on the insulating layer deposited last so that two electrical hollow-cylindrical conductors are located underneath a hollow-cylindrical insulator up to the inner pipe wall of the rod linkage. A further measure is taken at the point of transition of two drill pipes in such a manner that connectors, insulated electrically towards the outside, fluid-sealed and provided with electrical conductors in the interior, electrically connect the insulated electrical path of each such rod linkage to the insulated electrical path of the corresponding neighboring rod linkage. This also occurs via the electrically conductive hollow-cylindrical layers over several layers so that at least one insulated electrical path is produced continuously from an upper end of the drilling string to a lower end of the drilling string. The considerable vibrations and forces occurring at the screwed transition points of the rod linkage require an uninterrupted electrical at the connectors which can scarcely be achieved.
EP 1 434 063 A2 describes a “drill string telemetry system and method” with a telemetry system and a telemetry method for communicating information via a drilling rod linkage. The information of a sensor in the drill bit is modulated onto a carrier signal with the aid of a transmitter and a transmitter coil and transmitted at medium to high frequency from a first position via the drilling rod linkage to a second position, received and demodulated by means of a receiver and a receiver coil and processed further in the processor. By means of the arrangement, it is intended to directly detect technical drilling information which influences the quality of the drilling and extends the life of the drill bit device through knowledge of the current temperature of the motor bearing shells and the current rotational speeds of the motor drive shaft.